In many domains it is desirable to take measurements of physical phenomena and transmit the digital data acquired over a transmission line. Measuring the characteristics of earth formations is a good example. Measurements of the characteristics of different earth formations traversed by a borehole are generally carried out by lowering into the borehole a "tool" containing various types of sensing instrumentation. The tool is attached to a logging cable which is used both for holding the equipment and as an electrical medium for the transmission of data signals from the tool to a data receiver on the surface.
Most downhole data acquisition systems currently in use process and store the information thus gathered in digital form. A digital signal carrying that information has frequency components ranging from very high to very low (or d.c.) frequencies and is referred to as a baseband signal. Baseband signals cannot normally be transmitted over bandpass channels (i.e., those channels which transmit only a limited range of frequencies) such as logging cable due to pulse-shape and frequency distortion of the signal. Therefore, it is necessary to resort to modulation methods whereby the transmitter modulates a sinusoidal carrier waveform with the baseband signal, the modulated carrier being suitable for transmission over the bandpass channel. The uphole receiver then recovers the baseband signal by demodulation, the modulator-demodulator pair being referred to as a modem.
In order to reduce downtimes, it is typical to simultaneously lower into the borehole several tools in the same combination. The information gathered by the different tools must then be transmitted to the surface either by time or frequency multiplexing. When it is desired to increase the number of tools within a given combination and yet to have the same quantity of data transmitted per tool per unit of time, the data transmission rate must be increased. That rate, however, is limited by both the frequency characteristics of the logging cable as well as environmental constraints on the downhole transmitter.
The logging cable has a relatively narrow useable bandwidth of about 5 kHz to 90 kHz: however, it does have a high signal to noise ratio of about 30 dB. The downhole environment sometimes reaches temperatures of 175.degree. C. These high temperatures restrict the selection of analog and digital components, which eliminates many standard modulation techniques. Thus specialized techniques much be employed for implementing high data rate digital transmission systems in such environments. Further because of the harsh downhole environment and cost factors, it is desirable to eliminate as many components as possible from the transmitter and if possible, eliminate any microprocessors. There is a need, therefore, for a method and apparatus for transmitting digital data at high speeds over a bandpass channel. In particular, there is a need for transmitting log data at high data rates over logging cable.